Discharge lamp lighting device, illumination device, and projector

ABSTRACT

In a chopper circuit, output power is controllable with a direct current power source as a power source, and a smoothing capacitor is connected between output terminals of the chopper circuit. A polarity inversion circuit applies an alternating voltage to a high pressure discharge lamp with a voltage across the smoothing capacitor as a power source. The output power of the chopper circuit and an inversion frequency of the polarity inversion circuit are controlled by a control circuit based upon a terminal voltage of the smoothing capacitor, which is detected by a voltage detecting circuit. In the control circuit, a switch voltage is set for defining a range of voltages detected by the voltage detecting circuit, and the inversion frequency is changed in plural stages according to the magnitude relation between the detected voltage and the switch voltage. The inversion frequency corresponding to electric power applied to the high pressure discharge lamp is set with respect to each range of lamp voltages, to thereby inhibit occurrence of an arc jump.

TECHNICAL FIELD

The present invention relates to a discharge lamp lighting device, whichlights a high pressure discharge lamp for use as a light source of aliquid crystal projector and the like, an illumination device and aprojector.

BACKGROUND ART

There has recently been proposed a use of a high pressure discharge lampas a light source of a liquid crystal projector, an automobile headlightor the like. As shown in FIG. 21, a discharge lamp lighting device forlighting this kind of high pressure discharge lamp is typicallyconstituted such that: a voltage of a direct current power source(including a pulsating power source obtained by full-wave rectifying acommercial power source) E is stepped down by a step down type choppercircuit 1; an output voltage of the chopper circuit 1 is smoothed by asmoothing capacitor C1; a direct current voltage as a voltage across thesmoothing capacitor C1 is converted into an alternating voltage whosepolarity is to be alternated by a polarity inversion circuit 2 whichcomprises a full bridge circuit; and the alternating voltage outputtedfrom the polarity inversion circuit 2 is applied to a load circuitincluding a high pressure discharge lamp La. The load circuit comprisesa filter circuit consisting of a series circuit of a capacitor C2 and aninductor L2, and has a constitution where the high pressure dischargelamp La is connected in parallel with the capacitor C2. That is, arectangular wave voltage from which a high frequency element has beenremoved by the filer circuit is applied to the high pressure dischargelamp La.

The chopper circuit 1 has a serial circuit of a switching element Q1made of a metal-oxide semiconductor field-effect transistor (MOSFET) andan inductor L1, which have been inserted between the direct currentpower source E and the smoothing capacitor C1, and a diode D1 isconnected in parallel with the serial circuit of the inductor L1 and thesmoothing capacitor C1. The polarity of the diode D1 is determined suchthat energy which is stored in the inductor L1 when the switchingelement Q1 is ON is then discharged as a regeneration current throughthe smoothing capacitor C1 when the switching element Q1 is OFF.Further, in the illustrated example, a resistor R1 for detecting acurrent is inserted between the negative electrode of the direct currentpower source E and the anode of the diode D1. The terminal voltage ofthe smoothing capacitor C1 is parted by a voltage detecting circuit 3consisting of a serial circuit of two resistors R2 and R3, and a voltageacross the resistor R3 is outputted, as a voltage proportional to theterminal voltage of the smoothing capacitor C1, from the voltagedetecting circuit 3.

A polarity inversion circuit 2 is a circuit where four switchingelements Q2 to Q5 each made of a MOSET are bridge-connected, and aserial circuit of the switching elements Q2 and Q3 and a serial circuitof the switching elements Q4 and Q5 are each connected as an arm of thebridge circuit between each terminal of the smoothing capacitor C1. Aload circuit is connected between a connection point of the switchingelements Q2 and Q3 and a connection point of the switching elements Q4and Q5. That is, a state where the switching elements Q2 and Q5 are onwhile the switching elements Q3 and Q4 are off and a state where theswitching elements Q2 and Q5 are off while the switching elements Q3 andQ4 are on are controlled so as to be alternately repeated, whereby analternating voltage is applied to the load circuit. Since the loadcircuit includes the serial circuit of the capacitor C2 and the inductorL2, and a voltage across the capacitor C2 is applied to the highpressure discharge lamp La, the lamp current of the high pressuredischarge lamp La can be changed by changing a frequency (hereinafterreferred to as “inversion frequency”) for on/off of the switchingelements Q2 to Q5.

The on/off of the switching elements Q1 to Q5 included in the choppercircuit 1 and the polarity inversion circuit 2 are controlled by acontrol circuit 4. The control circuit 4 starts controlling theswitching elements Q1 to Q5 in the chopper circuit 1 and the polarityinversion circuit 2 when a lightning signal is inputted from an exteriorportion, and the control circuit 4 changes an output power of thechopper circuit 1 when an electric power switching signal S2 is inputtedfrom an external portion. Further, the control circuit 4 monitors, witha voltage across the resistor R1, a current corresponding to the lampcurrent of the high pressure discharge lamp La, and also monitors anoutput voltage of the voltage detecting circuit 3, to performpulse-width-modulation (PWM) control of the switching element Q1 of thechopper circuit 1 so as to maintain electric power instructed by theelectric power switching signal S2. Moreover, the control circuit 4outputs a control signal for turning the switching elements Q2 to Q5 onand off, and the control signal is provided to the switching elements Q2to Q5 through drivers 2 a and 2 b. An on/off duty ratio of the switchingelements Q2 to Q5 is here set to 50% so as to equally wear out twoelectrodes disposed in the high pressure discharge lamp La.

Incidentally, the high pressure discharge lamp La for use as a liquidcrystal projector or an automobile headlight has electrodes dose to oneanother and can thus be used as a point source, and it is known that, inthis kind of high pressure discharge lamp La, a phenomenon occurs wherea luminescent spot on the electrode, i.e. a radiant point of an electroncurrent when the electrode is on the cathode side, is not stabilized ina fixed position and moves disorderly. This phenomenon is called an arcjump, and when the arc jump occurs in a light source for a liquidcrystal projector, a luminescent spot is displaced with respect to anoptical system to be used along with the light source, causing a problemof variations in light amount on a screen. That is, a change in electricpower to be charged during lightening of the high pressure dischargelamp La leads to variations in temperature of or distance between theelectrodes, and further when a fan for air cooling is built in a housinglike a liquid crystal projector, a change in condition for air coolingleads to variations in temperature of or distance between theelectrodes. As thus described, when the state of the electrodes varies,a voltage across the electrodes varies, resulting in occurrence of anarc jump. Especially when the illuminating time of the high pressuredischarge lamp La becomes longer, the voltage across the electrodesincreases, and also when supply power to the high pressure dischargelamp La is switched in the lower electric power direction, the lampcurrent decreases to cause lowering of the electrode temperature,thereby making the arc jump tend to occur.

In a state where the high pressure discharge lamp La is stably on, thelamp current varies as the voltage across the smoothing capacitor C1 ischanged by PWM controlling the switching element Q1 of the choppercircuit 1. That is, the lamp current varies by changing either theon/off duty ratio of the switching element Q1 of the chopper circuit 1or the inversion frequency of the switching elements Q2 to Q5 of thepolarity inversion circuit 2. However, a knowledge has been obtainedthat there exists a relation for stabilizing the state of the electrodesof the high pressure discharge lamp La, between the voltage across thesmoothing capacitor C1 (which corresponds to the lamp voltage, asdescribed later) and the frequency of the alternating voltage to beapplied to the high pressure discharge lamp La. In other words, it hasbeen found that there exists an optimum value of the inversion frequencyaccording to the lamp voltage (the voltage across the smoothingcapacitor C1) to the polarity inversion circuit 2, as a condition forreducing variations in temperature of or distance between the electrodesto keep the electrodes in a stable state. Therefore, if the inversionfrequency and the lamp voltage of the polarity inversion circuit 2 incombination are optimum values, the occurrence of the arc jump issuppressed to reduce the wearing out of the electrodes, therebyextending the life of the high pressure discharge lamp La.

In the following, the relation between the lamp voltage and theinversion frequency in the polarity inversion circuit 1 is considered.Firstly considered is the case where the inversion frequency iscontrolled so as to be kept constant irrespective of the lamp voltage.The optimum value of the inversion frequency is here set to f1 in therange of lamp voltages from V1 to V2. When the inversion frequency iscontrolled so as to be kept at f1 irrespective of the lamp voltage asshown by A in FIG. 22, the inversion frequency f1 is the optimum valuein the range of lamp voltages from V1 to V2 as shown by B1, whereas theoptimum value of the inversion frequency is f2 in the range of lampvoltages lower than V1 as shown by B2, and the optimum value of theinversion frequency is f3 in the range of lamp voltages higher than V2as shown by B3, indicating that the inversion frequency is not theoptimum value in either range of lamp voltages. That is, when theinversion frequency is fixed, in the range of the lamp voltages from V1to V2, the state of the electrodes of the high pressure discharge lampLa is stabilized, allowing inhibition of the occurrence of the arc jump,whereas, when the lamp voltage is lower than V1 or higher than V2, theinversion frequency is deviated from the optimum value and the state ofthe electrodes of the high pressure discharge lamp La thus becomeunstable, leading to occurrence of the arc jump.

Next, considered is the case where the electric power switching signalS2 instructs switching of the electric power and the inversion frequencyof the polarity inversion circuit 2 is controlled so as to be keptconstant irrespective of the instructed electric power. As shown by A inFIG. 23, the optimum value of the inversion frequency is here set to f1in the range of lamp voltages from V1 to V2 when an electric power isP1. When the electric power is switched from P1 to P2, the lamp voltageof the polarity inversion circuit 2 varies and the lamp voltage of thehigh pressure discharge lamp La then varies to cause deviation of theelectrodes of the high pressure discharge lamp La from the stable state,leading to the shift of the optimum value of the inversion frequency tothe frequency f2 as shown by B in FIG. 23. However, since the inversionfrequency is here controlled so as to be kept constant irrespective ofthe electric power, the electrodes consequently become unstable to leadto occurrence of the arc jump.

As another example for the control, as shown in FIG. 24, there can alsobe considered a method of continuously changing the inversion frequencyof the polarity inversion circuit 2 according to the lamp voltage. Inthe illustrated example, the inversion frequency is f1 when the lampvoltage is V1, and the inversion frequency is f2 when the lamp voltageis V2. That is, it is considered that, since the lamp voltage isconstantly kept at the optimum value at inversion frequencies from f1 tof2 in the range of lamp voltages from V1 to V2, the state of theelectrodes is kept stable. However, since even slight variations in lampvoltage are followed by variations in inversion frequency, the dutyratio of the lamp current in the current waveform becomes different from50% as revealed from FIG. 25( a), which may raise a problem of unequalwearing out of the electrodes to thereby shorten the life of the highpressure discharge lamp La.

In order to solve this kind of problem, a constitution has been proposedwhere information corresponding to the distance between the electrodesis monitored by the lamp voltage, the inversion frequency is madeswitchable in two stages, a width of increase/decrease of the lampvoltage from an initial value is detected, and the inversion frequencyis increased when the lamp voltage is on the decrease and theincrease/decrease width is larger than a prescribed threshold, while theinversion frequency is decreased when the lamp voltage stopsincreasing/decreasing (see e.g. Patent Document 1: Japanese Patent No.3327895, p 10-11, FIG. 7)

DISCLOSURE OF INVENTION

In a technique described in Patent Document 1, the lamp voltage ismonitored for obtaining information corresponding to the distancebetween the electrodes, and an inversion frequency is controlled so asto keep the distance between the electrodes almost constant forinhibiting an arc jump. However, the technique described in PatentDocument 1 has difficulty in certainly detecting variations in state ofthe electrodes due to variations in temperature of the electrodes orcondition for air cooling, thus having a problem of being unable toinhibit the occurrence of the arc jump by this kind of cause.

The present invention was made in view of the above described matters,and has an object to set an inversion frequency corresponding to anelectric power applied to a high pressure discharge lamp in each rangeof lamp voltages, to provide a discharge lamp lighting device capable ofinhibiting occurrence of an arc jump caused by variations in temperatureof the electrodes or condition for air cooling, and further provide anillumination device and a projector.

The invention of claim 1 comprises: a direct current power source; achopper circuit capable of controlling output power by performing DC-DCconversion with the direct current power source as a power source; asmoothing capacitor connected between the output terminals of thechopper circuit; a polarity inversion circuit for performing DC-ACconversion with a voltage across the smoothing capacitor as a powersource; a high pressure discharge lamp to which an alternating voltageis applied by the polarity inversion circuit; a control circuit forcontrolling an output of the polarity inversion circuit as well asoutput power of the chopper circuit; and a voltage detecting circuit fordetecting a voltage corresponding to a lamp voltage of a high pressuredischarge lamp, characterized in that a switch voltage for defining arange of voltages detected by the voltage detecting circuit is set inthe control circuit, and the control circuit has a function ofcontrolling the polarity inversion circuit such that an inversionfrequency, at which the polarity of the lamp current of the highpressure discharge lamp is inverted according to the magnitude relationbetween the detected voltage and the switch voltage, is changed inplural stages.

The invention of claim 2 is characterized in that, in the invention ofclaim 1, the control circuit is capable of selecting an output of thechopper circuit from several stages, and has a function of changing theinversion frequency corresponding to selectable electric power.

The present invention of claim 3 is characterized in that, in theinvention of claim 2, the switch voltage is regularly set regardless ofthe selectable electric power.

The invention of claim 4 is characterized in that, in the invention ofclaim 2, at least one of the switch voltages is set to a different valuewith respect to different electric power.

The invention of the claim 5 is characterized in that, in the inventionof any one of claims 2 to 4, an equal inversion frequency is appliedimmediately after lightening of the high pressure discharge lamp until avoltage detected by the voltage detecting circuit reaches a prescribedvoltage, irrespective of the selectable electric power.

The invention of claim 6 is characterized in that, in the invention ofanyone of claims 2 to 4, an equal inversion frequency is appliedimmediately after lightening of the high pressure discharge lamp untilreaching a prescribed switch time, irrespective of the selectableelectric power.

The invention of claim 7 is characterized in that, in the invention ofany one of claims 1 to 4, hysteresis is added to the switch voltage.

The invention of claim 8 is characterized in that, in the invention ofany one of claims 1 to 4, the control circuit determines whether or notto change the inversion frequency once every prescribed number ofpolarity inversions of the lamp current of the high pressure dischargelamp.

The invention of claim 9 is characterized in that, in the invention ofany one of claims 1 to 4, the control circuit determines whether or notto change the inversion frequency upon at least every lapse of aprescribed fixed time.

The invention of claim 10 is characterized in that, in the invention ofany one of claims 1 to 4, the control circuit determines the magnituderelation between the voltage detected by the voltage detecting circuitand the switch voltage at fixed time intervals so as to determine, onceevery prescribed times of determinations, whether or not to change theinversion frequency according to whether the number of determinationssatisfying a prescribed magnitude relation is not less than or less thana prescribed number.

The invention of claim 11 is characterized in that, in the invention ofany one of claims 1 to 4, the control circuit takes a voltage detectedby the voltage detecting circuit every time the polarity of the lampcurrent of the high pressure discharge lamp inverts.

The invention of claim 12 is characterized in that, in the invention ofclaim 11, the control circuit takes a voltage detected by the voltagedetecting circuit after the lapse of a prescribed time from the polarityinversion of the lamp current of the high pressure discharge lamp.

The invention of claim 13 is characterized in that, in the invention ofclaim 1, in the control circuit, the inversion frequency is changed at atiming when the polarity of the lamp current of the high pressuredischarge lamp has inverted even times.

The invention of claim 14 is an illumination device, comprising thedischarge lamp lighting device according to claim 1.

The invention of claim 15 is a projector, comprising the discharge lamplighting device according to claim 1.

The invention of claim 16 is a projector, comprising: a discharge lamplighting device; a fan for air-cooling a high pressure discharge lamp;and a projector control device which receives a lamp voltage detected bythe discharge lamp lighting device and is capable of instructing, to thedischarge lamp lighting device, an inversion frequency at which thepolarity of the lamp current of the high pressure discharge lamp isinverted, characterized in that, the projector control device sets acontrol condition for air-cooling by the fan according to the lampvoltage received from the high pressure discharge lamp and instructs, tothe discharge lamp lighting device, an inversion frequency correspondingto the control condition.

The invention of claim 17, in the invention of claim 1, comprises an arcjump detecting means for detecting an arc jump which occurs in the highpressure discharge lamp, characterized in that, in the control circuit,a duty ratio of a lamp current waveform of the high pressure dischargelamp is set to a different value from 50% when the arc jump is detectedby the arc jump detecting means.

The invention of claim 18 is characterized in that, in the invention ofclaim 17, the number of polarity inversions of the lamp current isdefined to such a degree of number as to eliminate the arc jump during aperiod when the duty ratio of the lamp current waveform has been set toa different value from 50%.

The invention of claim 19 is characterized in that, in the invention ofclaim 17, a period when the duty ratio of the lamp current waveform hasbeen set to a different value from 50% is defined as a period when avalue detected by the arc jump detecting means, with which the arc jumpwas detected, is changed by a variation thereof for returning to theoriginal value.

The invention of claim 20 is characterized in that, in the invention ofclaim 18 or 19, the duty ratio of the lamp current waveform is changedwith time during a period when the duty ratio has been set to adifferent value from 50%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing an embodiment of the presentinvention.

FIG. 2 (a), FIG. 2 (b), FIG. 2 (c) and FIG. 2 (d) are operationexplanatory views showing Embodiment 1 of the present invention.

FIG. 3 (a) and FIG. 3 (b) are operation explanatory views showingEmbodiment 2 of the present invention.

FIG. 4 (a) and FIG. 4 (b) are operation explanatory views showingEmbodiment 3 of the present invention.

FIG. 5 (a) and FIG. 5 (b) are operation explanatory views showingEmbodiment 4 of the present invention.

FIG. 6 (a) and FIG. 6 (b) are the operation explanatory views same asabove.

FIG. 7 (a) and FIG. 7 (b) are the operation explanatory views same asabove.

FIG. 8 (a) and FIG. 8 (b) are operation explanatory views showingEmbodiment 5 of the present invention.

FIG. 9 (a) and FIG. 9 (b) are operation explanatory views showingEmbodiment 6 of the present invention.

FIG. 10 (a) and FIG. 10 (b) are operation explanatory views showingEmbodiment 7 of the present invention.

FIG. 11 (a) and FIG. 11 (b) are operation explanatory views showingEmbodiment 8 of the present invention.

FIG. 12 (a) and FIG. 12 (b) are operation explanatory views showingEmbodiment 9 of the present invention.

FIG. 13 (a) and FIG. 13 (b) are operation explanatory views showingEmbodiment 10 of the present invention.

FIG. 14 (a) and FIG. 14 (b) are operation explanatory views showingEmbodiments 7 to 10 of the present invention.

FIG. 15 (a) and FIG. 15 (b) are the operation explanatory views same asabove.

FIG. 16 is a schematic constitutional view showing Embodiment 11 of thepresent invention.

FIG. 17 (a) and FIG. 17 (b) are operation explanatory views showingEmbodiment 12 of the present invention.

FIG. 18 (a) and FIG. 18 (b) are the operation explanatory views same asabove.

FIG. 19 (a) and FIG. 19 (b) are operation explanatory views showingEmbodiment 13 of the present invention.

FIG. 20 is an operation explanatory view of another example ofEmbodiments 12 and 13 of the present invention.

FIG. 21 is a circuit diagram showing a conventional example.

FIG. 22 is the operation explanatory view same as above.

FIG. 23 is the operation explanatory view same as above.

FIG. 24 is the operation explanatory view same as above.

FIG. 25 (a) and FIG. 25 (b) are the operation explanatory views same asabove.

BEST MODE FOR CARRYING OUT THE INVENTION EMBODIMENT 1

A discharge lamp lighting device to be described in the followingembodiment basically has the constitution shown in FIG. 1, using thesame chopper circuit 1, polarity inversion circuit 2 and voltagedetecting circuit 3 as those in the conventional constitution shown inFIG. 21. A control circuit 4 is constituted using a microcomputer(abbreviated as “Micon”) 10, and an electric power instruction value S5is provided from the microcomputer 10 to a PWM control circuit 11 sothat the PWM control circuit 11 turns the switching element Q1 of thechopper circuit 1 on and off at a duty ratio according to the electricpower instruction value S5. In the PWM control circuit 11, a voltageacross a resistor R1 for detecting a current is monitored, and the dutyratio for the on/off of the switching element Q1 is increased anddecreased such that a current value detected as the voltage across theresistor R1 agrees with a target value specified as the electric powerinstruction value S5. Further, the microcomputer 10 outputs a controlsignal which determines an inversion frequency as a frequency for theon/off of the switching elements Q2 to Q5 with respect to a full bridgecontrol circuit 12, and in the full bridge control circuit 12, a controlsignal is produced which determines a timing for the on/off of theswitching elements Q2 to Q5 that are disposed in each arm of thepolarity inversion circuit 2. The control signal outputted from the fullbridge control circuit 12 is provided to the switching elements Q2 to Q5through drivers 2 a and 2 b.

A microcomputer “M37450”, manufactured by Mitsubishi ElectricCorporation, can for example be used as the microcomputer 10, and adriver “IR2111”, manufactured by International Rectifier Corporation,can for example be used as the drivers 2 a and 2 b. The microcomputer 10has a function of operating and stopping the PWM control circuit 11 andthe full bridge control circuit 12 with the lightning signal S1 providedfrom the external portion, and houses an A/D conversion circuit forconverting a voltage (voltage proportional to the terminal voltage ofthe smoothing capacitor C1) detected by the voltage detection circuit 4into a digital value. Further, upon receiving the electric powerswitching signal S2, the microcomputer 10 can switch a supply power tothe high pressure discharge lamp La in two or more stages, and theelectric power instruction value S5 is then determined by electric powerselected by the electric power switching signal S2 and a voltageobtained from the voltage detecting circuit 3. That is, selectableelectric power is previously stored in the microcomputer 10, and eachelectric power is alternatively selected every time the electric powerswitching signal S2 is inputted. The microcomputer 10 is also providedwith a function of dividing the selected electric power by the detectedvoltage for determining a current value, and then providing this currentvalue as the electric power instruction value S5 to the PWM controlcircuit 11. As apparent from this operation, when electric power to besupplied to the high pressure discharge lamp La is selected in themicrocomputer 10, the relation between the terminal voltage of thesmoothing capacitor C1 and the current detected by the resistor R1 iscontrolled such that the electric power is set to the selected electricpower value, and the terminal voltage of the smoothing capacitor C1corresponds to the lamp voltage while the current detected by theresistor R1 corresponds to the lamp current.

On the other hand, in the present embodiment, the inversion frequency ofthe control signal to be provided to the full bridge control circuit 12is defined with the range of voltages detected in the voltage detectingcircuit 3 as a parameter. That is, using a ROM [EEPROM] built in themicrocomputer 10, the lamp voltage (i.e. the voltage detected in thevoltage detecting circuit 3) is sectioned into plural ranges, in each ofwhich a V/F conversion table corresponding to an inversion frequency isset, and the inversion frequency is determined by checking the voltagedetected in the voltage detecting circuit 3 with reference to the V/Fconversion table. At least one switch voltage, at which the inversionfrequency is switched, is set, thus making the inversion frequencyswitchable in two or more stages. In the V/F conversion table, as shownin FIG. 2( a), when one switch voltage V1 is for example used, theinversion frequency is set to f1 in the voltage range lower than theswitch voltage V1, and the inversion frequency is set to f2 (>f1) in thevoltage range not lower than the switch voltage V1. Further, as shown inFIG. 2( b), when two switch voltages V1 and V2 (V1<V2) are for exampleused, the inversion frequency is set to f1 in the voltage range lowerthan the switch voltage V1, the inversion frequency is set to f2 (>f1)in the voltage range not lower than the switch voltage V1 and lower thanthe switch voltage V2, and further, the inversion frequency is set to f3(>f2) in the voltage range not lower than the switch voltage V2. It isto be noted that the lower limit of the voltage detected in the voltagedetecting circuit 3 is 0 V while the upper limit of the same is avoltage obtained by multiplying the voltage of the direct current powersource E by a partial pressure ratio which is determined by theresistors R2 and R3.

It is to be noted that the relation of the polarity inversionfrequencies is not restricted to the example of FIG. 2( b), but may beset to f3>f1>f2 as shown in FIG. 2( c), or f1>f2>f3 as shown in FIG. 2(d). Further, the number of lamp voltage ranges is not restricted tothree, but may be larger. That is, the polarity inversion frequency isset so as to be an optimum value in each given lamp voltage range.

An external control signal S3 for determining the on/off of theswitching elements Q2 to Q5 of the polarity inversion circuit 2 can alsobe inputted in the microcomputer 10, and when the external controlsignal S3 is inputted, a rectangular wave signal inputted as theexternal control signal S3 is applied to the full bridge control circuit12 irrespective of the inversion frequency having been determined in theV/F conversion table. That is, when the external control signal S3 isinputted, the on/off frequency and duty ratio) of the switching elementsQ2 to Q5 of the polarity inversion circuit 2 is determined by theexternal control signal S3.

Moreover, upon receiving the lightening signal S1, the microcomputer 10is activated, and during lightning of the high pressure discharge lampLa, a rectangular wave signal for determining a duty ratio according tothe voltage of the smoothing capacitor C1 (which corresponds to the lampvoltage) is outputted as a voltage information signal S4 from themicrocomputer 10. For example, when the terminal voltage of thesmoothing capacitor C1 varies from 0 V to 255 V, the voltage informationsignal S4 is a rectangular wave signal corresponding 0 to 255 V to dutyratios of 0 to 100%.

Accordingly, the inversion frequency is set to a relatively lowfrequency f1 in the range of lamp voltages, detected as terminalvoltages of the smoothing capacitor C1, lower than V1, and as in theconventional constitution, the lamp current decreases when the lampvoltage becomes higher than V1 with the inversion frequency kept fixedto f1, leading to lower temperatures of the electrodes of the highpressure discharge lamp La than in the case where the lamp voltage isbelow V1, which makes the arc jump tend to occur. As opposed to this, inthe constitution of the present embodiment, the inversion frequencyvaries to f2, which is higher than f1, when the lamp voltage becomeshigher than V1, allowing inhibition of a decrease in temperature of theelectrodes of the high pressure discharge lamp La, and thereby it ispossible to prevent the occurrence of the arc jump. Further, theoccurrence of the arc jump can further be inhibited with greatercertainty when two switch voltages are set rather than one switchvoltage is set.

EMBODIMENT 2

Embodiment 1 represents the constitution where the inversion frequencyis determined using the lamp voltage alone as a parameter, whereas inthe present embodiment, the electric power selected by the electricpower switching signal S2 is also used as a parameter for determiningthe inversion frequency, along with the lamp voltage. That is, as thesupply power to the high pressure discharge lamp La becomes smaller, thelamp current decreases to lower the temperatures of the electrodes ofthe high pressure discharge lamp La, and hence the inversion frequencyis controlled so as to become higher as the supply power becomessmaller. In order to achieve this constitution, a V/F conversion tableis set with respect to each electric power selected by the electricpower switching signal S2, and when one switch voltage, V1, is forexample used, as in FIG. 3( a), the inversion frequencies (f1, f2) areset to be relatively low as shown by A1 and A2 in FIG. 2 with respect tolarge electric power P1, while the inversion frequencies (f1′, f2′) areset to be relatively high as shown by B1 and B2 in FIG. 2 with respectto small electric power P2. When two switch voltages, V1 and V2, areused and the electric power is selectable from three stages: furtherlarge electric power P1; intermediate electric power P2; and smallelectric power P3, the inversion frequencies are respectively set tocharacters like (f1, f2, f3), (f1′, f2′, f3′) and (f1″, f2″, f3″) withrespect to the electric power P1 to P3 as shown by A1 to A3(corresponding to the electric power P1), B1 to B3 (corresponding to theelectric power P2), and C1 to C3 (corresponding to the electric powerP3) in FIG. 3( b). In the present embodiment, the switch voltage V1 (V2)is fixed irrespective of the selected electric power, therebyfacilitating creation of the V/F conversion table. It is to be notedthat, as described above, the respective characters starting with A, Band C correspond to the electric power P1, P2 and P3, and theserelations are applied to each of embodiments below.

In the present embodiment, it is possible to correspond not only to thecase where the temperatures of the electrodes of the high pressuredischarge lamp La decrease due to the variations in lamp voltage, but tothe case where the temperatures of the electrodes decrease according tothe selected supply power, allowing significant suppression of theoccurrence of the arc jump. Other constitutions and functions are thesame as those of Embodiment 1.

EMBODIMENT 3

In Embodiment 2, the switch voltage V1 (V2) is fixed irrespective of theelectric power selected by the electric power switching signal S2,whereas in the present embodiment, the switch voltage is changed withrespect to the selected electric power. That is, when the supply poweris selected from the two stages and one switch voltage is set withrespect to each stage of the electric power, as shown in FIG. 4( a), theinversion frequency is switched to f1 before the switch voltage V1 andto f2 (>f1) after the switch voltage V1, as shown by A1 and A2, withrespect to large electric power P1, while the inversion frequency isswitched to f1′ before the switch voltage V1′ (<V1) and to f2′ (>f1′)after the switch voltage V1′, as shown by B1 and B2, with respect tosmall electric power P2. In such a manner, the switch voltage is set tobe lower as the electric power is smaller.

When the supply power is selected from the three stages of P1 to P3(P1>P2>P3) and two switch voltages are set with respect to each stage ofthe electric power, the inversion frequencies may be respectively set tocharacters like (f1, f2, f3), (f1′, f2′, f3′) and (f1″, f2″, f3″) withrespect to the electric power P1 to P3, as shown by A1 to A3(corresponding to the electric power P1), B1 to B3 (corresponding to theelectric power P2), and C1 to C3 (corresponding to the electric powerP3) in FIG. 4( b). Two switch voltages have been set with respect toeach stage of the electric power P1 to P3, and the switch voltage is setto be lower as the electric power is smaller. That is, the switchvoltages are V1 and V2 with respect to the large electric power P1, theswitch voltages are V1′ and V2′ (V1>V1′, V2>V2′) with respect to theintermediate electric power P2, and the switch voltages are V1″ and V2″(V1′>V1″, V2′>V2″) with respect to the small electric power P3.

In the constitution of the present embodiment, since not only theinversion frequency is changed but the switch voltage is also changedaccording to the supply power, it is possible to make a setting withwhich the occurrence of the arc jump is further prevented. It is to benoted that, although every switch voltage is changed with respect toeach stage of the electric power in the foregoing example as shown inFIG. 4( b), part of the switch voltages may be equal even with respectto different stages of electric power. In short, at least one switchvoltage may be different with respect to each stage of the electricpower. The other constitutions and operations are the same as those ofEmbodiment 1.

EMBODIMENT 4

In the present embodiment, the inversion frequencies are equalized inthe range of low lamp voltages irrespective of the selected electricpower as in Embodiment 1 and, out of the inversion frequency and theswitch voltage, at least the inversion frequency is changed with respectto each stage of the electric power in the range of relatively high lampvoltages as in Embodiment 2 or 3. That is, as shown in FIG. 5( a), inthe voltage range lower than the switch voltage V0, the inversionfrequency is set to f1 irrespective of the selected electric power, andin the voltage range not lower than the switch voltage V0 and lower thanthe switch voltage V1, the inversion frequency is kept at f1 withrespect to the large electric power while being raised to f1′ withrespect to the small electric power. Moreover, in the voltage range notlower than the switch voltage V2 which is higher than V1, both theinversion frequencies with respect to the large power and small powerare raised to f2 and f2′, respectively.

As shown in FIG. 5( a), with the V/F conversion table previously set,the electric power and the lamp current vary with respect to the lampvoltage as shown in FIGS. 6( a) and (b), respectively. That is, withrespect to the large electric power, the lamp current becomes constantin the voltage range from 0V to the vicinity of the switch voltage V1,and the electric power becomes constant in the voltage range higher thana voltage that is slightly lower than the switch voltage V1. Further,with respect to the small electric power, the lamp current becomesconstant in the voltage range from 0V to the degree exceeding the switchvoltage V0, and the electric power becomes constant in the voltage rangehigher than a voltage that is slightly higher than the switch voltageV0. In short, the voltage as a switching point between the constantcurrent control and the constant electric power control becomes lower asthe electric power is smaller. Such a setting can be employed incontrolling shift of a constant current controlling period to a constantelectric power controlling period, immediately after lightening of thehigh pressure discharge lamp La. That is, even when the electric poweris different, the inversion frequency is not changed for a period fromthe lightening to at least the switch voltage V0, and it is therebypossible to control a constant current immediately after the lightningirrespective of the selected electric power.

FIG. 5( a) represents an example in which the electric power is madeselectable from two stages and two switch voltages are set with respectto the small electric power, while in the case where the electric poweris made selectable from three stages, two switch voltages are set withrespect to the large electric power and three switch voltages are setwith respect to each of the other electric power, the example shown inFIG. 5( b) is preferably applied. When the V/F conversion table is setas shown in FIG. 5( b), the electric power and the lamp current varywith respect to the lamp voltage as in FIG. 7( a) and FIG. 7( b). Otherconstitutions and operations are the same as those of Embodiment 1.

EMBODIMENT 5

In Embodiment 4, the inversion frequencies are equally set in the rangeof lamp voltages lower than the switch voltage V0 even with respect todifferent stages of the electric power selected by the electric powerswitching signal S2, whereas in the present embodiment, the inversionfrequencies are equally set irrespective of the electric power selectedby the electric power switching signal S2 until the time for lightningthe high pressure discharge lamp La reaches a prescribed switch time,and the inversion frequencies are changed according to the selectedelectric power when the illuminating time passes the switch time. Thatis, the inversion frequencies are equalized irrespective of the electricpower selected by the electric power switching signal S2 as in FIG. 8(a) until the time for lightning the high pressure discharge lamp Lareaches the switch time. However, even during this period, the inversionfrequency is changed according to the lamp voltage range. Here, theinversion frequency is set to f1 in the voltage range lower than theswitch voltage V1, and the inversion frequency is set to f2, which ishigher than f1, in the voltage range not lower than the switch voltageV1. Further, when the illuminating time passes the switch time, theinversion frequencies are made different according to the electric powerselected by the electric power switching signal S2 as in FIG. 8( b). Inthe illustrated example, with respect to the large electric power, theinversion frequency is switched between f1 and f2 (>f1) across theswitch voltage V1 as shown by A1 and A2, whereas with respect to thesmall electric power, the inversion frequency is switched between f1′and f2′ (>f1′) across the switch voltage V1 as shown by B1 and B2.

Although the example is represented above in which the electric power ismade selectable from two stages and only one switch voltage is set, thenumber of switch voltages can be further increased, and the electricpower may be made selectable from three or more stages. Otherconstitutions and operations are the same as those of Embodiment 1.

EMBODIMENT 6

Since each of the foregoing embodiments represents the constitutionwhere the inversion frequencies are switched across the switch voltage,when the lamp voltage varies in the vicinity of the switch voltage, theinversion frequency may unstably vary to cause an unstable operation. Inthe present embodiment, therefore, hysteresis is added to the relationbetween the lamp voltage and the inversion frequency. Namely, as shownin FIG. 9( a), two higher and lower stages of the switch voltages V1 hand V1 b (<V1 h) are set, and when the inversion frequency is set to f1and the lamp voltage exceeds the higher switch voltage V1 h, theinversion frequency is increased to f2, whereas when the inversionfrequency is set to f2 and the lamp voltage falls below the lower switchvoltage V1 b, the inversion frequency is decreased to f1. Such anoperation allows elimination of unnecessary switching of the inversionfrequency. FIG. 9( b) represents the case of making the inversionfrequencies different according to the electric power, where the sameoperation is performed as in FIG. 9( a). Other constitutions andoperations are the same as those of Embodiment 1.

EMBODIMENT 7

In Embodiment 6, the hysteresis is added to the relation between thelamp voltage and the inversion frequency to stabilize the operation atthe time of switching the inversion frequency, whereas in the presentembodiment, time intervals, at which whether or not to switch theinversion frequency is determined, are set to be relatively large so asto stabilize the operation at the time of switching the inversionfrequency. Namely, the time intervals at which the lamp voltage isdetected for determining the inversion frequency are defined by thenumber of polarity inversions of the lamp current, and for example, thelamp voltage is detected once every eight times of polarity inversionsof the lamp current as shown in FIG. 10( a) so as to determine whetherthe lamp voltage is lower than the switch voltage V1 or not lower thanthe switch voltage V1 as shown in FIG. 10( b). The number of polarityinversions of the lamp current is practically not counted by monitoringthe lamp current, but determined based upon the number of controlsignals outputted from the microcomputer 10.

In the illustrated example, the case is assumed where the inversionfrequency is switchable in two stages, f1 and f2, with only one switchvoltage set, and as shown in FIG. 10( a), at a time t1, the lowerinversion frequency f1 is selected since the lamp voltage is lower thanthe switch voltage V1; then at a time t2, a time point when the polarityhas inverted eight times after the time t1, the higher inversionfrequency f2 is selected since the lamp voltage is higher than theswitch voltage V1; and at a time t3 and a time t4 thereafter, the lowerinversion frequency f1 is selected since the lamp voltage is lower thanthe switch voltage V1.

As thus described, since the lamp voltage for use in determining whetheror not to switch the inversion frequency is detected every time thenumber of polarity inversions of the lamp current reaches a prescribednumber, the time intervals at which the lamp voltage is detected becomerelatively long, thereby enabling prevention of unstable switching ofthe inversion frequency Although the case of setting the inversionfrequency in two stages is described as an example in the presentembodiment, the same technique is applicable to the case where theinversion frequency is selectable from three or more stages. Moreover,although the lamp voltage is determined for determining whether or notto change the inversion frequency once every eight times of polarityinversions of the lamp current, the number of inversions is notparticularly limited, and can be appropriately set so long as being sucha degree that the time elapsed for the inversions is relatively shortand the inversion frequency is not switched unstably. Otherconstitutions and operations are the same as those of Embodiment 1.

EMBODIMENT 8

In Embodiment 7, the lamp voltage is detected for determining whether ornot to change the inversion frequency once every prescribed number ofpolarity inversions of the lamp current, and thus the time intervals atwhich the lamp voltage is detected vary depending upon the selectedinversion frequency. The present embodiment represents a constitutionwhere the variations in time intervals are reduced more than the case ofEmbodiment 7 while the time intervals at which the lamp voltage isdetected are made relatively long, in the same manner as in Embodiment7.

Namely, in the present embodiment, at the time point when a prescribedfixed time T has elapsed after the detection of the lamp voltage and thelamp current polarity varies in a specific direction, the subsequentdetection of the lamp voltage is performed. In the example shown inFIGS. 11( a) and 11(b), using the lamp voltage detected at a timing whenthe lamp current polarity inverts from the negative to the positive at atime t1 as shown in FIG. 11( a), when the detected lamp voltage is lowerthan the switch voltage V1 as shown in FIG. 11( b), the inversionfrequency is set to f1. Next, the lamp voltage is detected at a time t2when the lamp current polarity inverts from the negative to the positivefor the first time after the lapse of a prescribed fixed time T from thetime t1. In the illustrated example, at the time t2, the inversionfrequency is set to the higher one, f2, since the lamp voltage is higherthan the switch voltage V1. At a time t3 when the polarity inverts fromthe negative to the positive after the lapse of the fixed time T fromthe time t2, and also at a time t4 when the polarity inverts from thenegative to the positive after the lapse of the fixed time T from thetime t3, the inversion frequency is set to the lower one, f1, since thelamp voltage is lower than the switch voltage V1.

As thus described, since the lamp voltage for use in determining whetheror not to switch the inversion frequency is detected at the timing whenthe lamp current polarity inverts after the lapse of the fixed time T,the time intervals at which the lamp voltage is detected becomerelatively long, thereby enabling prevention of unstable switching ofthe inversion frequency. Further, although the case of setting theinversion frequency in two stages is described as an example in thepresent embodiment, the same technique is applicable to the case wherethe inversion frequency is selectable from three or more stages. Otherconstitutions and operations are the same as those of Embodiment 1.

EMBODIMENT 9

In the present embodiment, the lamp voltage is detected at prescribedtime intervals, as well as the magnitude relation between the lampvoltage and the switch voltage being determined, and at the time pointwhen the lamp voltage has been detected the prescribed number of times,based upon the magnitude relation between the lamp voltage and theswitch voltage in each of the determinations, a majority decision ismade to adopt the magnitude relations the number of which is larger soas to determine the inversion frequency, and if the inversion frequencyneeds to be changed, the change is made at the subsequent timing of thepolarity inversion of the lamp current.

Here described as an example is the case where one switch voltage, V1,is used while the inversion frequency is changed in two stages, f1 andf2(>f1), and the inversion frequency is determined once every five timesof determinations of the magnitude relation between the lamp voltage andswitch voltage. Namely, as shown in FIG. 12( b), the magnitudes of thelamp voltage and the switch voltage V1 are compared at fixed timeintervals, and in the illustrated example, in a state where theinversion frequency is f1, the lamp voltage is larger than the switchvoltage V1 three times out of the first five times of determinations,the lamp voltage is lower than the switch voltage V1 three times out ofthe subsequent five times of determinations, and the lamp voltage islower than the switch voltage V1 five times out of the furthersubsequent five times of determinations. That is, the inversionfrequency is changed from f1 to f2 according to the result of the firstfive times of determinations, the inversion frequency is changed to f1according to the result of the subsequent five times of determinations,and the inversion frequency is kept at f1 according to the result of thefurther subsequent five times of determinations. The timing for changingthe inversion frequency is set to a timing at which the lamp currentpolarity is switched from the negative to the positive, as shown in FIG.12( a).

As thus described, in the present embodiment, since the magnituderelation between the lamp voltage and the switch voltage is regularlydetermined so as to determine, by the majority decision at prescribedtime intervals, whether or not to switch the inversion frequency, thetime intervals at which the lamp voltage is detected become relativelylong, thereby enabling prevention of unstable switching of the inversionfrequency. Although, here, the number of times of determinations, basedupon which the majority decision is made, is set to five, it is notparticularly limited. However, it is preferable to set the number oftimes of determinations, based upon which the majority decision is made,to an odd number when the inversion frequency is selected from the twostages, and in this case, the inversion frequency can be prevented frombecoming indeterminate. Further, whether or not to switch the inversionfrequency may be determined not necessarily by the majority decision butby whether the number of determinations satisfying either condition forthe magnitude relation out of the prescribed number of determinations isnot less than or less than a prescribed number. Moreover, although thecase of setting the inversion frequency in two stages is described as anexample in the present embodiment, the same technique is applicable tothe case where the inversion frequency is selectable from three or morestages. Other constitutions and operations are the same as those ofEmbodiment 1.

EMBODIMENT 10

In Embodiment 9, the magnitude relation between the lamp voltage and theswitch voltage is determined at fixed time intervals, whereas in thepresent embodiment, as shown in FIGS. 13( a) and 13(b), the magnituderelation between the lamp voltage and the switch voltage is determinedevery time the lamp current (cf. FIG. 13( a)) polarity inverts, and amajority decision is made once every fixed number (eight times in theillustrated example) of polarity inversions. Further, in onedetermination of the magnitude relation between the lamp voltage and theswitch voltage, the lamp voltage is obtained prescribed times (threetimes in the illustrated example) and an average value of the obtainedvoltages is used as the lamp voltage. Here, the inversion frequency isset to f2 when the lamp voltage exceeds the switch voltage V1 (cf. FIG.13( b)) not less than five times out of eight times of determinations,and the inversion frequency is set to f1 when the lamp voltage exceedsthe switch voltage V1 less than five times. It is to be noted that, thenumber of determinations of the magnitude relation between the lampvoltage and the switch voltage is not limited to eight, and the numberof lamp voltages whose average value is to be used as the lamp voltageis not necessarily three. Other constitutions and functions are the sameas those of Embodiment 9.

In foregoing Embodiments 7 to 10, the comparison between the lampvoltage and the switch voltage is required. Here, as shown in FIG. 14(b), the lamp voltage appears not to vary in broad perspectiveimmediately after the polarity inversion of the lamp current as shown inFIG. 14( a) and FIG. 15( a), but as shown in FIG. 15( b), the lampvoltage varies in reality immediately after the polarity inversion.Therefore, a desirable timing for detecting the lamp voltage is notimmediately after the polarity inversion of the lamp current, but afterthe lapse of a prescribed time T1 from the polarity inversion as shownin FIG. 15( b).

Moreover, in each of Embodiments 7 to 10, the number of polarityinversions at each inversion frequency is controlled so as to be an evennumber. This is for equalizing the wearing out of the electrodes of thehigh pressure discharge lamp La so as to extend the life of the highpressure discharge lamp La.

The discharge lamp lighting device of each of foregoing Embodiments 1 to10 is usable for a variety of lightening devices using the high pressuredischarge lamp La as a light source, and is used for a variety ofprojectors using the high pressure discharge lamp La as a light source,such as a liquid crystal projector.

EMBODIMENT 11

As shown in FIG. 16, the present embodiment represents a constitutionalexample of a liquid crystal projector using a discharge lamp lightingdevice 20 having the foregoing constitution, and light distribution ofthe high pressure discharge lamp La as a light source is controlled by areflector 21. Each of constituents of the liquid crystal projector,including the discharge lamp lighting device 20, is controlled by aprojector control circuit 22, and between the projector control circuit22 and the discharge lamp lighting device 20, the voltage informationsignal S4 corresponding to the lamp voltage is sent from the dischargelamp lighting device 20 while the electric power switching signal S2 andthe external control signal S3 are sent from the projector controlcircuit 22. A rectangular wave signal is here used for the externalcontrol signal S3 as well as voltage information signal S4.

The lamp voltage is information which reflects the temperature of thehigh pressure discharge lamp La, and in the projector control circuit22, a control condition for a fan 23 for cooling the high pressuredischarge lamp La is determined based upon the voltage informationsignal S4, and the optimum inversion frequency is determined accordingto the control condition for the fan 23. In the projector controlcircuit 22, the external control signal S3 corresponding to thedetermined inversion frequency is provided to the discharge lamplighting device 20, and upon receiving the external control signal S3,the discharge lamp lighting device 20 controls the polarity inversioncircuit 2.

Namely, with the constitution of the present embodiment adopted, it ispossible not only to adjust the inversion frequency but to control thefan 23 for cooling the high pressure discharge lamp La. Otherconstitutions and operations are the same as those of Embodiment 1.

EMBODIMENT 12

In each of the foregoing embodiments, in order to prevent one electrodeof the high pressure discharge lamp La from being worn out more than theother electrode, the polarity inversion circuit 2 is driven so as to setthe duty ratio to 50%. As opposed to this, in the present embodiment,the arc jump is detected, and the duty ratio of the lamp currentwaveform is shifted from 50% when the arc jump is detected. For thedetection of the arc jump, an arc jump determining means can beconstituted, for example, such that the lamp current is monitored andthe occurrence of the arc jump is determined when the average value ofthe lamp currents decreases. For example, as shown in FIG. 17( b), adetected amount relative to the presence or absence of the arc jump isobtained in the arc jump determination means, and the detected amount iscompared with a threshold Th to detect the presence or absence of theoccurrence of the arc jump. In the illustrated example, the duty ratioof the lamp current is 50% when the arc jump is not detected, and theduty ratio is changed to an appropriate value Dv that is different from50% after the detection of the arc jump.

With this technique adopted, it is possible to control the temperatureof the electrode in which the arc jump has occurred, so as to be raisedat the time of the occurrence of the arc jump, thereby resulting inreduction in occurrence of the arc jump.

Moreover, when the arc jump is detected and the duty ratio is thenchanged to Dv, as shown in FIG. 18( a), the arc jump can be eliminatednormally by several times (about ten times) of polarity inversions ofthe lamp current, and therefore the duty ratio is returned to 50% aftersuch a degree of number of polarity inversions as to be slightly largerthan the above-mentioned number of polarity inversions. That is, theduty ratio is returned to the original ratio of 50%, not depending uponthe comparison between the amount detected by the arc jump detectingmeans and the threshold Th, but upon the number of polarity inversions.

With this technique, it is possible to control the temperature of theelectrode of the high pressure discharge lamp La so as to be furtherraised even when the arc jump, having occurred due to variations inelectrode temperature, is eliminated by the change in duty ratio,thereby permitting inhibition of the occurrence of another arc jump. Theother constitutions and operations are the same as those of Embodiment1.

EMBODIMENT 13

In Embodiment 12, the duty ratio is controlled so as to be returned tothe original value after the polarity of the lamp current has beeninverted several times after the detection of the elimination of the arcjump, whereas in the present embodiment, as shown in FIG. 19( b), usinga variation Δ V of a value detected by the arc jump detecting means inexceeding the threshold Th, the duty ratio is returned to 50% when thevalue detected by the arc jump detecting means varies by the variation ΔV with respect to the threshold th during a period when the duty ratioof the lamp current waveform has been changed to Dv as shown in FIG. 19(a). Other constitutions and operations are the same as those ofEmbodiment 12.

Although in each of Embodiments 12 and 13, the duty ratio is keptconstant during a period when the duty ratio of the lamp currentwaveform has been changed due to the detection of the arc jump, the dutyratio may be changed with time during the period when the duty ratio hasbeen changed to Dv, as shown in FIG. 20. In the illustrated example, theduty ratio is largest immediately after the change therein, and thengradually decreased with time. In this constitution, it is possible toheat the electrode to eliminate the arc jump even when the arc jump hasoccurred in either one of the pair of electrodes of the high pressuredischarge lamp La.

INDUSTRIAL APPLICABILITY

As thus described, according to the constitution of the presentinvention, the relation between the lamp voltage and the inversionfrequency can be kept appropriate according to the state of electrodesof the high pressure discharge lamp, consequently allowing inhibition ofthe occurrence of the arc jump in the high pressure discharge lamp.

1. A discharge lamp lighting device, comprising: a direct current powersource; a chopper circuit capable of controlling an output power byperforming DC-DC conversion with the direct current power source as apower source; a smoothing capacitor connected between output terminalsof the chopper circuit; a polarity inversion circuit for performingDC-AC conversion with a voltage across the smoothing capacitor as apower source; a high pressure discharge lamp to which an alternatingvoltage is applied by the polarity inversion circuit; a control circuitfor controlling an output of the polarity inversion circuit as well asan output power of the chopper circuit; and a voltage detecting circuitfor detecting a voltage corresponding to a lamp voltage of a highpressure discharge lamp, wherein a switch voltage for defining a rangeof voltages detected by the voltage detecting circuit is set in thecontrol circuit, and the control circuit has a function of controllingthe polarity inversion circuit such that an inversion frequency, atwhich the polarity of the lamp current of the high pressure dischargelamp is inverted according to a magnitude relation between the detectedvoltage and the switch voltage, is changed in plural stages.
 2. Thedischarge lamp lighting device according to claim 1, wherein saidcontrol circuit is capable of selecting an output of said choppercircuit from several stages, and has a function of changing saidinversion frequency corresponding to the selectable electric power. 3.The discharge lamp lighting device according to claim 2, wherein saidswitch voltage is regularly set irrespective of the selectable electricpower.
 4. The discharge lamp lighting device according to claim 2,wherein at least one of said switch voltages is set to a different valuewith respect to different electric power.
 5. The discharge lamp lightingdevice according to claim 2, wherein an equal inversion frequency isapplied immediately after lightening of said high pressure dischargelamp until a voltage detected by the voltage detecting circuit reaches aprescribed voltage, irrespective of the selectable electric power. 6.The discharge lamp lighting device according to claim 2, wherein anequal inversion frequency is applied immediately after lightening ofsaid high pressure discharge lamp until reaching a prescribed switchtime, irrespective of the selectable electric power.
 7. The dischargelamp lighting device according to claim 1, wherein said switch voltageis added with a hysteresis.
 8. The discharge lamp lighting deviceaccording to claim 1, wherein said control circuit determines whether ornot to change said inversion frequency once every prescribed number ofpolarity inversions of the lamp current of said high pressure dischargelamp.
 9. The discharge lamp lighting device according to claim 1,wherein said control circuit determines whether or not to change saidinversion frequency upon at least every lapse of a prescribed fixedtime.
 10. The discharge lamp lighting device according to claim 1,wherein said control circuit determines the magnitude relation betweenthe voltage detected by the voltage detecting circuit and the switchvoltage at fixed time intervals so as to determine, once everyprescribed times of determinations, whether or not to change theinversion frequency according to whether the number of determinationssatisfying a prescribed magnitude relation is not less than or less thana prescribed number.
 11. The discharge lamp lighting device according toclaim 1, wherein said control circuit takes a voltage detected by saidvoltage detecting circuit every time the polarity of the lamp current ofsaid high pressure discharge lamp inverts.
 12. The discharge lamplighting device according to claim 11, wherein, said control circuittakes a voltage detected by the voltage detecting circuit after thelapse of a prescribed time from the polarity inversion of the lampcurrent of said high pressure discharge lamp.
 13. The discharge lamplighting device according to claim 1, wherein said control circuitchanges said inversion frequency at a timing when inversions of thepolarity of the lamp current of said high pressure discharge lamp hasoccurred even times.
 14. An illumination device, comprising thedischarge lamp lighting device according to claim
 1. 15. A projector,comprising the discharge lamp lighting device according to claim
 1. 16.The discharge lamp lighting device according to claim 1, comprising anarc jump detecting means for detecting an arc jump which occurs in saidhigh pressure discharge lamp, wherein said control circuit sets a dutyratio of a lamp current waveform of said high pressure discharge lamp toa different value from 50% when the arc jump is detected by the arc jumpdetecting means.
 17. The discharge lamp lighting device according toclaim 16, wherein the number of polarity inversions of the lamp currentis defined to such a degree of number as to eliminate the arc jumpduring a period when the duty ratio of said lamp current waveform hasbeen set to a different value from 50%.
 18. The discharge lamp lightingdevice according to claim 17, wherein the duty ratio of the lamp currentwaveform is changed with time lapse during a period when the duty ratiohas been set to a different value from 50%.
 19. The discharge lamplighting device according to claim 16, wherein a period when the dutyratio of said lamp current waveform has been set to a different valuefrom 50% is defined as a period when a value detected by the arc jumpdetecting means, with which the arc jump was detected, is changed by avariation thereof for returning to an original value.
 20. The dischargelamp lighting device according to claim 19, wherein the duty ratio ofthe lamp current waveform is changed with time lapse during a periodwhen the duty ratio has been set to a different value from 50%.